Pad design for reliability enhancement in packages

ABSTRACT

A package includes a corner, a device die having a front side and a backside, and a molding material molding the device die therein. A plurality of redistribution lines is on the backside of the device die. The plurality of redistribution lines includes a plurality of metal pads. A polymer layer contacts the plurality of metal pads. A plurality of openings is formed in the polymer layer, with the plurality of metal pads aligned to and exposed to the plurality of openings. The plurality of openings includes a corner opening that is elongated and an additional opening farther away from the corner than the corner opening. The additional opening is non-elongated.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/613,997, entitled “Pad Design for Reliability Enhancement inPackages,” filed Feb. 4, 2015, which claims the benefit of the followingprovisionally filed U.S. Patent application: Application Ser. No.62/011,432, filed Jun. 12, 2014, and entitled “Integrated CircuitPackage Pad and Method of Forming Same;” which applications are herebyincorporated herein by reference.

BACKGROUND

In the packaging of integrated circuits, there are various types ofpackaging methods and structures. For example, in a conventionalPackage-on-Package (POP) process, a top package is bonded to a bottompackage. The top package and the bottom package may also have devicedies packaged therein. By adopting the PoP process, the integrationlevel of the packages is increased.

In an existing PoP process, the bottom package, which includes a devicedie bonded to a package substrate, is formed first. A molding compoundis molded to the package substrate, wherein the device die is molded inthe molding compound. The package substrate further includes solderballs formed thereon, wherein the solder balls and the device die are ona same side of the package substrate. The solder balls are used forconnecting the top package to the bottom package.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of a package in accordancewith some embodiments;

FIG. 2 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein corner openings are elongated connectors;

FIG. 3 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein a plurality of elongated openings is distributed to each corner;

FIG. 4 illustrates the top view of a package and non-solder openings ofthe package in accordance with some embodiments, wherein elongatedopenings and non-elongated openings are distributed depending on theirdistances to a neutral-stress point of the package;

FIG. 5 illustrates a top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein inner openings adjacent to the corners of an underlying devicedie are elongated;

FIG. 6 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein inner openings adjacent to the corners of an underlying devicedie group are elongated;

FIG. 7 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein a plurality of inner openings adjacent to each corner of anunderlying device die is elongated;

FIG. 8 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein a plurality of inner openings adjacent to each corner of anunderlying device die group is elongated;

FIG. 9 illustrates the top view of a package and openings in a topdielectric layer of the package in accordance with some embodiments,wherein openings in corner rectangular regions are elongated;

FIGS. 10A and 10B illustrate top views of some exemplary elongatedopenings;

FIGS. 11A and 11B illustrate top views of some exemplary non-elongatedopenings in accordance with some embodiments;

FIGS. 12A and 12B illustrate the exemplary shapes of elongated openingsand their respective underlying metal pads in accordance with someembodiments; and

FIG. 13 illustrates the metal pads on a package that is bonded to thepackage with elongated openings.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

An integrated fan-out package and the structure for improving thereliability of the package are provided in accordance with variousexemplary embodiments. The variations of the embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements.

FIG. 1 illustrates a cross-sectional view of package 400 in accordancewith some exemplary embodiments of the present disclosure. Package 400includes bottom package 100 and top package 200 over and bonded tobottom package 100. Both bottom package 100 and top package 200 arepre-formed, and are then bonded to each other to form a package onpackage structure. In accordance with some embodiments of the presentdisclosure, bottom package 100 includes device dies 102 (including 102Aand 102B), with the front sides of device dies 102 facing down andbonded to Redistribution Lines (RDLs) 112. In alternative embodiments,bottom package 100 includes a single device die or more than two devicedies. Device dies 102 may include semiconductor substrates 108, andintegrated circuit devices (such as active devices, which includetransistors, for example) 104 at the front surface (the surface facingdown) of semiconductor substrates 108. Device dies 102 may include logicdies such as Central Processing Unit (CPU) dies, Graphic Processing Unit(GPU) dies, mobile application dies, or the like.

Device dies 102 are molded in molding material 120, which surroundsdevice dies 102. Molding material 120 may be a molding compound, amolding underfill, a resin, or the like. The bottom surface 120A ofmolding material 120 may be leveled with the bottom ends of device dies102. The top surface 120B of molding material 120 may be level with orhigher than back surfaces 108A of semiconductor substrates 108. Inaccordance with some embodiments of the present disclosure, backsurfaces 108A of semiconductor substrates 108 are overlapped bydie-attach films 110, which adhere device dies 102 to the overlyingdielectric layer 118 and RDLs 116. Device dies 102 further include metalpillars 106 (which may include copper pillars) in contact with, andbonded to, RDLs 112.

Bottom package 100 may include Front-side RDLs 112 underlying devicedies 102 and back-side RDLs 116 overlying device dies 102. The term“front-side RDL” indicates that the respective RDLs are on the frontside of device dies 102, and the term “back-side RDL” indicates that therespective RDLs are on the back side of device dies 102. Front-side RDLs112 are formed in dielectric layers 114, and back-side RDLs 116 areformed in dielectric layers 118. RDLs 112 and 116 may be formed ofcopper, aluminum, nickel, alloys thereof, or multi-layers thereof. Inaccordance with some embodiments of the present disclosure, dielectriclayers 114 and 118 are formed of inorganic materials such as siliconoxide, silicon nitride, silicon oxynitride, or the like. In alternativeembodiments, dielectric layers 114 and 118 are formed of organicmaterials such as polymers, which may include polybenzoxazole (PBO),benzocyclobutene (BCB), polyimide, or the like.

Through-Vias 122 are formed to penetrate through molding material 120.In accordance with some embodiments of the present disclosure,through-vias 122 have top surfaces level with the top surface of moldingmaterial 120 and bottom surfaces level with the bottom surface ofmolding material 120. Through-Vias 122 electrically connect front-sideRDLs 112 and device dies 102A and 102B to back-side RDLs 116.Through-Vias 122 may also be in physical contact with some of front-sideRDLs 112 and back-side RDLs 116.

Electrical connectors 124, which are formed of a non-solder metallicmaterial(s), are formed at the bottom surface of bottom package 100. Inaccordance with some embodiments of the present disclosure, electricalconnectors 124 are metal pads. In alternative embodiments, electricalconnectors 124 include metal pillars such as copper pillars. Throughoutthe description, electrical connectors 124 are referred to as metal pads124, although they may have forms other than metal pads. Metal pads 124may comprise copper, aluminum, nickel, palladium, gold, or multi-layersthereof. Solder regions 126 are attached to the bottom surfaces of metalpads 124 and bond bottom package 100 to package component 300. In someexemplary embodiments, Under Bump Metallurgies (UBMs) 127 are formed atthe bottom surface of package component 100, with solder regions 126attached on. In alternative embodiments, no UBM is formed, and solderregions 126 are in contact with metal pads 124. Package component 300may include a Printed Circuit Board (PCB), a package, or another type ofpackage component.

The back-side RDLs 116 includes some metal pads 150. In accordance withsome embodiments, metal pads 150 are in the topmost RDL layer in packagecomponent 100. Polymer layer 152 is formed over RDLs 116 and dielectriclayers 118. Dielectric layer 152 may be formed of a polymer such as PBOor other organic or inorganic materials. Throughout the description,dielectric layer 152 is referred to as polymer layer 152 although it mayalso be formed of a dielectric material other than polymer. Inaccordance with some embodiments, tape 154 is over and attached todielectric layer 152. Tape 154 is used to provide protection andreinforcement to the underlying structure. Furthermore, laser marks 156may be formed in tape 154. Laser marks 156 are recesses/openings in tape154 and may be formed through laser. The top-view shape of laser marks156 may include letters, numbers, graphics, and/or the like. Hence,laser marks 156 may be used for identification purposes. In alternativeembodiments, tape 154 is not formed, and polymer layer 152 is the toplayer of package component 100.

Openings 158 are formed in polymer layer 152 and tape 154, and metalpads 150 are exposed to opening 158. Solder regions 206 have theirbottom portions filling openings 158, with solder regions 206 in contactwith metal pads 150.

Top package 200 is bonded to bottom package 100. In accordance with someembodiments of the present disclosure, top package 200 includes packagesubstrate 202 and device die 204, which is bonded to package substrate202. The bonding of device die 204 to package substrate 202 may beachieved through wire bonding, flip-chip bonding, or the like. Solderregions 206 bond top package 200 to bottom package 100. Furthermore,solder regions 206 are in contact with metal pads 208 at the bottomsurface of package component 200. Accordingly, solder regions 206 havetheir top surface in contact with metal pads 208 and bottom surfaces incontact with the top surfaces of metal pads 150.

FIG. 2 illustrates a schematic top view of portions of bottom package100, wherein openings 158 in polymer layer 152 (and tape 154, if any,FIG. 1) are illustrated. Openings 158 include elongated openings 158Aand non-elongated openings 158B. In FIGS. 2 through 9, circles are usedto schematically represent non-elongated openings 158B, and ovals areused to schematically represent elongated openings 158A. FIGS. 10A and10B illustrate a top view of some exemplary elongated opening 158A.Elongated opening 158A has length L1 measured in lengthwise direction Xand width W1 measured in widthwise direction Y, which is perpendicularto lengthwise direction X. Length L1 is greater than width W1. Elongatedopening 158A includes longer axis 134 in lengthwise direction X andshorter axis 136 in widthwise direction Y. Throughout the description,an elongated opening 158A is an opening whose length-to-width ratioL1/W1 is greater than threshold ratio Lt that is greater than 1.Threshold ratio Lt may be greater than about 1.2, 1.6, or 2.0.

Elongated opening 158A may have various shapes including, but notlimited to, an oval shape, a rectangular shape, an elongated octagonalshape, or the like. For example, FIG. 10A illustrates an exemplaryelongated opening 158A, which includes two half circles 130 connected toopposite edges of rectangle 128. FIG. 10B illustrates an elongatedhexagon. It is appreciated that other elongated shapes other than whathave been discussed may also be used.

FIGS. 11A and 11B illustrate the top views of exemplary non-elongatedopenings 158B. Throughout the description, a non-elongated opening 158Bdoes not have length L2 and width W2 observably different from eachother. Alternatively, a non-elongated opening 158B has width W2 smallerthan length L2, with length-to-width ratio L2/W2 being smaller than thethreshold ratio Lt, which may be smaller than about 1.2 or about 1.1 inaccordance with some exemplary embodiments. For example, FIGS. 11A and11B illustrate non-elongated openings 158B, which have a circularbottom-view shape and a hexagonal bottom-view shape, respectively.

Although not shown in FIGS. 10A, 10B, 11A, and 11B, the top views ofsolder regions 206 (FIG. 1) are defined by, and may be the same as, thetop-view shapes of the respective underlying openings 158. Hence, solderregions 206 may also include elongated solder regions (in the top viewof package component 100) and non-elongated solder regions.

Referring back to FIG. 2, bottom package 100 includes four corners 138.The corner openings 158, which are closer to the respective corners 138than all other openings, are elongated openings 158A. Other openings 158that are farther away from the respective corners 138 than the corneropenings 158A are non-elongated openings 158B. In accordance with someembodiments of the present disclosure, there may be more than oneelongated opening 158A at each corner 138. For example, as shown in FIG.3, there are three elongated openings 158A at each corner 138.

FIGS. 3 and 4 illustrate the top views of bottom package 100 andopenings 158 in accordance with alternative embodiments. In the topview, bottom package 100 has neutral-stress point 140, which is thepoint substantially free from stresses from all lateral directions thatare parallel to the bottom surface of package 100. At neutral-stresspoint 140, the lateral stresses from opposite directions are cancelledout. The lateral stresses are the stresses parallel to the top andbottom surfaces of package component 100 in FIG. 1. In accordance withsome embodiments of the present disclosure, neutral-stress point 140 isat or close to the center of bottom package 100 (in the top view). Thedistance between each of openings 158 and neutral-stress point 140 isreferred to as a Distance to Neutral Point (DNP) of the respectiveopening 158, wherein the distance of an opening 158 is measured from apoint of the opening 158 that is closest to neutral-stress point 140.For example, DNPs DNP1 and DNP2 are illustrated as examples in FIG. 3.

As shown in FIGS. 3 and 4, a circle 142 is drawn with neutral-stresspoint 140 as the center, wherein circle 142 has radius r. In accordancewith the embodiments of the present disclosure, all openings 158 withDNPs equal to or smaller than radius r are designed as non-elongatedopenings 158B, and all openings 158 with DNPs greater than radius r aredesigned to be elongated openings 158A. As illustrated in FIG. 3, ifradius r is large, then the elongated openings include corner openings.In these embodiments, each row includes at least one (or more) openingthat is a non-elongated opening. In FIG. 4, radius r is reduced, andhence an entire edge-row or edge-column of openings 158 whose DNPs aregreater than radius r are elongated, while the openings 158 with theDNPs equal to or smaller than radius r are non-elongated. In theembodiments in FIGS. 3 and 4, the elongated openings 158A include corneropenings.

Radius r is determined based on the stresses suffered by solder regions206 and metal pads 150 (FIG. 1) and may be based on measurement resultsfrom packages and/or simulation results. In some embodiments, radius ris selected by ensuring the stresses suffered by all solder regions 206and metal pads 150 in circle 142 (FIGS. 3 and 4) are lower than apre-determined threshold stress, while the stresses suffered by at leastsome solder regions 206 and metal pads 150 outside circle 142 are higherthan the threshold stress.

In accordance with some embodiments, openings 158 are distributed as anarray that is distributed throughout package 100, as shown in FIGS. 3through 5. In alternative embodiments, openings 158 are distributedclose to the peripheral regions of package 100, and are not in the innerregions. For example, FIGS. 2 and 3 schematically illustrate dashedrectangular regions 159. In these embodiments, openings 158 will beformed outside of rectangular regions 159, and will not be formed insiderectangular regions 159.

FIG. 5 illustrates the top view of some components in bottom package 100and openings 158 in accordance with yet alternative embodiments. Devicedie 102 is illustrated in the top view. Device die 102 includes corners146. Solder regions that are close to the corners 146 suffer from higherstresses than other nearby solder regions and hence are more prone tofailure. In accordance with some embodiments of the present disclosure,the inner openings 158 (which are not edge openings and not corneropening of package 100) that overlap (also refer to FIG. 1) device die102 and are also closest to corners 146 are elongated openings 158A,while other openings 158 (except the openings 158A discussed in theembodiments in FIGS. 2 through 4) are non-elongated openings 158B. Inaccordance with some embodiments of the present disclosure, the innerelongated openings 158A are surrounded by non-elongated openings 158B.

FIG. 6 illustrates some components in the top view of bottom package 100and openings 158 in accordance with yet alternative embodiments. Inthese embodiments, a plurality of device dies 102 is closely locatedfrom each other to form device die group 148. Device die group 148includes corners 146. In accordance with some embodiments of the presentdisclosure, there are four corner regions 147, each at a corner 146.There are four openings 158 in each of corner regions 147, wherein thefour openings 158 are closest to the respective corner 146. Each of thefour openings 158 in each corner region 147 includes one opening thatoverlaps die group 148 and three openings 158 that do not overlap diegroup 148. All four openings 158 in each corner region 147 are elongatedopenings 158A, while other openings 158 in the regions surrounding eachof corner regions 147 are non-elongated openings 158B.

There is also a plurality of edge regions 149 that overlaps the edges ofdie group 148. In each of edge regions 149, there are two rows ofopenings 158, with each of the two rows extending in the directionparallel to the respective edge of die group 148. One of the rows ofopenings 158 overlaps die group 148, and the other row of openings 158does not overlap die group. In accordance with some embodiments of thepresent disclosure, edge openings 158 in edge regions 149 arenon-elongated openings 158B. In alternative embodiments, edge openings158 in edge regions 149 are elongated openings 158A.

In accordance with some embodiments as shown in FIGS. 5 and 6, the inneropenings 158 that are overlapped by device die group 148, whose openings158 are also closest to corners 146, are elongated openings 158A, whileother openings 158 (except the openings 158A discussed in theembodiments in FIGS. 2 through 4) are non-elongated openings 158B. Inaccordance with some embodiments of the present disclosure, the innerelongated openings 158A are surrounded by non-elongated openings 158B.

In the above-discussed embodiments referring to FIG. 6, die group 148includes two dies. In alternative embodiments, die group 148 may includea single die or more than two dies. For example, as shown in FIG. 5,when die group 148 includes a single die, then the die corners 146 as inFIG. 6 will be the four corners of the single die.

FIGS. 7 and 8 illustrate the top views of bottom package 100 andopenings 158 in accordance with yet alternative embodiments. Theseembodiments are similar to the embodiments in FIGS. 5 and 6 except thatmore openings 158 that are close to the corners 146 of, and overlaps,device dies 102 or device die group 148 are elongated openings 158A,which are surrounded by non-elongated openings 158B. Furthermore, FIG. 7illustrates a single device die 102, with elongated openings 158A beingdistributed close to the corners 146 of device die 102. FIG. 8illustrates a device die group 148, with elongated openings 158A beingdistributed close to the corners 146 of device die group 148. Inaccordance with some embodiments in FIGS. 5 through 8, an innerelongated opening 158A may be fully, or partially, overlapped by theoverlying device die 102 or device die group 148.

FIG. 9 illustrates the design of openings 158 in accordance with yetalternative embodiments. In these embodiments, four corner regions 160of bottom package 100 are defined, each extending from one of corners138 inwardly. The four corner regions 160 may have rectangular shapesand may have sizes the same as each other. The openings 158 insidecorner regions 160 are designed as elongated opening 158A. The openings158 outside corner regions 160 may be designed as non-elongated openings158B or may be designed as elongated opening 158A.

In some embodiments as in FIG. 9, circle 142 is also drawn according tosimulation or experiment results. The radius of circle 142 may be small,and hence some of openings 158 that are outside of corner regions 160are also outside of circle 142. Accordingly, as shown in FIG. 6, some ofopenings 158 (marked as 158″) that are outside of the circle 142 arealso elongated openings 158A, while the openings 158 that are outside ofcorner regions 160, but inside circle 142, are non-elongated openings158B.

FIGS. 12A and 12B illustrate the top views of some exemplary elongatedopenings 158A and the respective metal pad 150 (also refer to FIG. 1)that are underlying and in contact with the elongated openings 158A. Inaccordance with some embodiments, metal pad 150 has a non-elongatedshape such as a circle (as shown in FIG. 12A), a square, a hexagon, anoctagon, or the like. In alternative embodiments, as shown in FIG. 12B,metal pad 150 may also have an elongated shape that fits the shape ofthe respective elongated opening 158A. Furthermore, the long axis ofmetal pad 150 may be parallel to, and may overlap, the long axis of therespective elongated opening 158A. By designing metal pad 150 to beelongated, the reliability of the respective package may be furtherimproved.

In the above-discussed embodiments, openings 158 include elongatedopenings 158A and non-elongated openings 158B. As shown in FIG. 1,openings 158 (including elongated openings 158A and non-elongatedopenings 158B) are bonded to metal pads 208 through solder regions 206.To fit the shapes of elongated openings 158A and non-elongated openings158B, metal pads 208 in package component 200 may also be designed tohave elongated metal pads 208A overlapping and bonded to elongatedopenings 158A and elongated metal pads 208B overlapping and bonded tonon-elongated openings 158B. An exemplary bottom view of metal pads 208Aand 208B are schematically illustrated in FIG. 13. In accordance withsome embodiments of the present disclosure, each of the elongatedopenings 158A in FIGS. 2 through 9 corresponded to, and are overlappedby and bonded to, the respective overlying elongated metal pads 208A,and each of the non-elongated openings 158B in FIGS. 2 through 9corresponded to, and are overlapped by, the respective overlyingnon-elongated metal pads 208B. The bottom shapes of elongated metal pads208A and non-elongated metal pads 208B and their respective positionsare similar to the shape of the corresponding elongated openings 158Aand non-elongated openings 158B in FIGS. 2 through 9, and hence are notillustrated.

Throughout the embodiments of the present disclosure, as shown in FIGS.2 through 9, elongated openings 158A are centripetal. In accordance withsome embodiments, the longer axis 134 (FIGS. 10A and 10B) of elongatedcentripetal openings 158A extend toward the neutral-stress point 140(FIGS. 2 through 4), which may be at, or at least close to, the centerof bottom package 100 (in the top view). Alternatively stated, thelonger axis 134 (FIGS. 10A and 10B) of the centripetal elongatedopenings 158A pass through the center of bottom package 100, or atleast, the longer axis 134 of the centripetal elongated openings 158Aare closer to the center of bottom package 100 than the respectiveshorter axis 136.

The embodiments of the present disclosure have some advantageousfeatures. By designing centripetal elongated openings, the solderregions in the centripetal elongated openings can endure higher stresseswithout failure than the solder regions in non-elongated openings. Thelocations of the centripetal elongated openings are selected accordingto the stresses suffered by the solder regions. Simulation resultsindicated that when the corner openings of the bottom package 100 arecentripetal elongated openings, the respective package fails after 769thermal cycles in the reliability test. In comparison, when the corneropenings of the bottom package 100 are non-elongated openings, therespective package fails after 604 thermal cycles. When the corneropenings of the bottom package 100 are elongated openings with thewidthwise directions extending toward the neutral-stress point, therespective bottom package fails after 574 thermal cycles. These resultsindicate that a package with centripetal elongated openings haveimproved reliability and can endure more thermal circles before theyfail.

In accordance with some embodiments of the present disclosure, a packageincludes a corner, a device die having a front side and a backside, anda molding material molding the device die therein. A plurality ofredistribution lines is on the backside of the device die. The pluralityof redistribution lines includes a plurality of metal pads. A polymerlayer contacts the plurality of metal pads. A plurality of openings isdisposed in the polymer layer, with the plurality of metal pads alignedto and exposed to the plurality of openings. The plurality of openingsincludes a corner opening that is elongated and an additional openingfarther away from the corner than the corner opening. The additionalopening is non-elongated.

In accordance with alternative embodiments of the present disclosure, apackage includes at least one first dielectric layer, a first pluralityof redistribution lines in the at least one first dielectric layer, adevice die over and electrically coupled to the first plurality ofredistribution lines, a molding material molding the device die therein,a through-via penetrating through the molding material, at least onesecond dielectric layer over the device die, and a plurality of metalpads over the molding material. The plurality of metal pads iselectrically coupled to the through-via and the device die. A polymerlayer is over the plurality of metal pads. A plurality of openings is inthe polymer layer, wherein the plurality of openings includes anelongated opening and a non-elongated opening. A plurality of solderregions extends into the plurality of openings to contact a respectiveone of the plurality of metal pads.

In accordance with yet alternative embodiments of the presentdisclosure, a package includes a device die, a through-via coplanar withthe device die, and a molding material molding the first device die andthe through-via therein. A plurality of metal pads is disposed on a sideof the molding material and electrically coupled to the device die andthe through-via. A polymer is in contact with the plurality of metalpads. A plurality of openings is in the polymer and aligned to theplurality of metal pads. The plurality of openings includes a corneropening that is elongated and centripetal, and an inner opening that iselongated and centripetal. The plurality of openings further includes aplurality of non-elongated openings surrounding the inner opening. Aplurality of solder regions is disposed in the plurality of openings. Apackage is bonded to the plurality of metal pads through the pluralityof solder regions.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a first package comprising:a corner; a device die comprising a front side and a backside; anencapsulating material encapsulating the device die therein; a firstplurality of redistribution lines on the backside of, and overlying, thedevice die, wherein the first plurality of redistribution linescomprises a first metal pad, a second metal pad and a third metal pad,wherein the first metal pad has a round top-view shape, and the firstmetal pad is non-elongated; a polymer layer overlying the first metalpad, the second metal pad, and the third metal pad, the polymer layercomprising a first opening exposing a first portion of the first metalpad, a second opening exposing a first portion of the second metal pad,and a third opening exposing a first portion of the third metal pad;wherein the first opening is an elongated opening adjacent to thecorner, the second opening is an elongated opening adjacent to the firstopening, and the third opening is a non-elongated opening farther awayfrom the corner than the first opening; and a through-via penetratingthrough the encapsulating material, wherein the through-via comprises afirst end and a second end, with the first end being between the secondend and the polymer layer, wherein the first end has substantiallystraight sidewalls, and the second end having curved sidewalls, andwherein portions of the second end farther away from the first end arenarrower than portions of the second end closer to the first end.
 2. Thepackage of claim 1, wherein the polymer layer covers a second portion ofeach of the first metal pad, the second metal pad, and the third metalpad.
 3. The package of claim 2, wherein the second portion of each ofthe first metal pad, the second metal pad, and the third metal pad formsa full ring encircling the first portions of the first metal pad, thesecond metal pad, and the third metal pad, respectively.
 4. The packageof claim 1 further comprising: a second package; and a first solderregion, a second solder region, and a third solder region extending intothe first opening, the second opening, and the third opening,respectively, to bond the first package to the second package.
 5. Thepackage of claim 4, wherein the second package comprises an additionalmetal pad in contact with the first solder region, wherein theadditional metal pad is elongated.
 6. The package of claim 5, wherein ina top view of the first package and the second package, a long axis ofthe first opening is parallel to a long axis of the additional metalpad.
 7. The package of claim 1, wherein the device die comprises a metalpillar on the front side, wherein the metal pillar is in contact withone of the first plurality of redistribution lines.
 8. The package ofclaim 1, wherein the first package has a neutral-stress point in a topview of the first package, wherein: all openings that are in the polymerlayer, have solder regions extending into, and have Distances to NeutralPoint (DNPs) greater than a radius, are elongated; and all openings thatare in the polymer layer, have solder regions extending into, and haveDNPs equal to or smaller than the radius are non-elongated.
 9. Thepackage of claim 1, wherein the polymer layer covers a ring-shaped outerportion of the first metal pad, and a portion of the first metal pad notcovered by the polymer layer has an elongated shape.
 10. The package ofclaim 1, wherein the first end of the first through-via is coplanar witha first surface of the encapsulating material.
 11. The package of claim10, wherein the second end of the first through-via is further coplanarwith a second surface of the encapsulating material.
 12. A packagecomprising: a first device die; a through-via; an encapsulating materialencapsulating the first device die and the through-via therein; aplurality of metal pads on a side of the encapsulating material andelectrically coupling to the first device die and the through-via,wherein each of the plurality of metal pads comprises a first portionand a second portion encircling the first portion; a polymer layer incontact with the second portion of each of the plurality of metal pads;a plurality of solder regions in contact with the first portions of theplurality of metal pads to form a plurality of interfaces, wherein theplurality of interfaces comprises: a corner interface, wherein thecorner interface is elongated in a top view of the package; a firstinner interface, wherein the first inner interface is elongated in thetop view, and wherein the corner interface and the first inner interfaceare centripetal; and a plurality of non-elongated interfaces surroundingthe first inner interface, wherein an edge row of the plurality ofinterfaces includes a non-elongated interface viewed in the top view,with the edge row being a row closest to an edge of the first device diethan other rows; and a package bonded to the plurality of metal padsthrough the plurality of solder regions.
 13. The package of claim 12further comprising a second inner interface among the plurality ofinterfaces, wherein the second inner interface is immediately next to,and is surrounded by, the plurality of non-elongated interfaces, withthe second inner interface being elongated.
 14. The package of claim 12,wherein the first inner interface comprises a portion aligned to atleast a part of the first device die.
 15. The package of claim 12further comprising a second device die in the encapsulating material,wherein the first device die and the second device die are in a diegroup, and wherein each of corners of the die group is adjacent to anelongated interface that is among the plurality of interfaces, withinner corners of the first device die and second device die inside thedie group having no adjacent elongated interfaces.
 16. The package ofclaim 12, wherein at least one of the plurality of non-elongatedinterfaces overlaps the first device die.
 17. A package comprising: afirst package comprising: a corner; a device die comprising a front sideand a backside; an encapsulating material encapsulating the device dietherein; a first plurality of redistribution lines on the backside of,and overlying, the device die, wherein the first plurality ofredistribution lines comprises a first metal pad, a second metal pad anda third metal pad; a polymer layer overlying the first metal pad, thesecond metal pad, and the third metal pad, the polymer layer comprisinga first opening exposing a first portion of the first metal pad, asecond opening exposing a first portion of the second metal pad, and athird opening exposing a first portion of the third metal pad, andwherein the first opening is an elongated opening adjacent to thecorner, the second opening is an elongated opening adjacent to the firstopening, and the third opening is a non-elongated opening farther awayfrom the corner than the first opening; a plurality of dielectric layerson the front side of the device die; a second plurality ofredistribution lines in the plurality of dielectric layers; a pluralityof Under-Bump Metallurgies (UBMs), each comprising a first portionextending into a bottom dielectric layer in the plurality of dielectriclayers to contact one of the second plurality of redistribution lines,and a second portion underlying the bottom dielectric layer; and athrough-via penetrating through the encapsulating material, wherein thethrough-via comprises a first end and a second end, with the first endbeing between the second end and the polymer layer, wherein the firstend has substantially straight sidewalls, and the second end havingcurved sidewalls, and wherein portions of the second end farther awayfrom the first end are narrower than portions of the second end closerto the first end.
 18. The package of claim 17, wherein the first metalpad has a round top-view shape, and the first metal pad isnon-elongated.
 19. The package of claim 18, wherein a second portion ofthe first metal pad forms a full ring encircling the first portions ofthe first metal pad.
 20. The package of claim 17 further comprising: asecond package; and a first solder region, a second solder region, and athird solder region extending into the first opening, the secondopening, and the third opening, respectively, to bond the first packageto the second package.